How many gas vents into 6" flue?

Max number of gas appliance vents connected to 6" stainless steel flexible flue liner? There are 2 small boilers and 2 50-gal water heaters manifolded into a 6" flue. What is the maximum capacity of such a flue in BTUs/hr or any other measure? Also, how should multiple appliance vents be manifolded to the flue? I know the smaller vent should be connected above the larger vent, but in this case they are all the same size.

The gas code has a table of about 6 or 7 pages. Total btus and length are a part of the calculations. If you have a doubt, refer it to a qualified HVAC tech to evaluate and make the call.